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Operating Systems

Operating Systems. Lesson 12. HTTP vs HTML. HTML: hypertext markup language Definitions of tags that are added to Web documents to control their appearance HTTP: hypertext transfer protocol The rules governing the conversation between a Web client and a Web server. HTML.

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Operating Systems

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  1. Operating Systems Lesson 12

  2. HTTP vs HTML • HTML: hypertext markup language • Definitions of tags that are added to Web documents to control their appearance • HTTP: hypertext transfer protocol • The rules governing the conversation between a Web client and a Web server

  3. HTML <html><body> <h1>My First Heading</h1> <p>My first paragraph.</p> <img src="hackanm.gif" width="45“ height="45" /></body> </html>

  4. HTTP • HTPP is a text-based protocol: uses text to represent its control messages • HTTP is a asymmetric: there are a client and server • HTTP is stateless: a session consist of single request/response pair • HTTP servers usually listen to incoming TCP connection on port 80

  5. URL

  6. HTTP Request

  7. HTTP Response

  8. HTTP request format Cr=‘\r’ lf=‘\n’

  9. HTTP response format

  10. Static vs. Dynamically Generated HTML pages

  11. Passing Parameters to the Web Server • HTML: • <form name="input" action=“form.asp" method="get"> <input type="text" name="user" /> <input type="submit" value="Submit" /></form> • Browser: • HTTP Request: • GET form.asp?user=tom HTTP/1.1 • Common Gateway Interface(CGI) • Standard for passing query parameters to the scripts/applications • Parameters(query string) via environment variable

  12. Some HTTP Request Methods • GET: retrieve information identified by the URL. • HEAD: retrieve meta-information about the URL • POST: send information to a URL and retrieve result. HTTP

  13. Response Status Line HTTP-Version Status-Code Message • Status Code is 3 digit number (for computers) • 1xx Informational • 2xx Success • 3xx Redirection • 4xx Client Error • 5xx Server Error • Message is text (for humans) HTTP

  14. Example Status Lines HTTP/1.0 200 OK HTTP/1.0 301 Moved Permanently HTTP 1/0 304 Not Modified HTTP/1.0 400 Bad Request HTTP/1.0 500 Internal Server Error HTTP

  15. Some Request Headers • User-Agent • Identification of the browser software • Accept • MIME types that browser is ready to accept • If-Modified-Since • Date of cached reqource

  16. Some Response Headers • Content-Type: • Type of the returned content • MIME identifier • text/html, video/mpeg • Content-Length • Size of the content in bytes • Last-Modified • Date of last modification

  17. Persistent Connection • HTTP 1.0 TCP connection is closed after each request/response exchange • HTTP 1.1 allows keeping TCP connection for next request/response • Browser: request header • Connection:Keep-Alive • Default in HTTP 1.1

  18. Cookies • Need a state over HTTP • Cookie Mechanism • Server instructs browser to keep a named value and send it back to the server along with next request • Cookie Expiration Policy • Session cookie: forget cookie after browser is closed • Expiration time: forget after prescribed time

  19. Cookie Headers • Server (Response Header) • Set-Cookie: name_a=value • Client (request Header) • Cookie: name_a=value; name_b=value

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